<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>HTMLDOM</title>
</head>
<body>

    <div id="div1">
        div
    </div>

<script>
    var div1 = document.getElementById("div1");
    var innerHTML = div1.innerHTML;
    div1.innerHTML += "<input type='text'>";
</script>

</body>
</html>